His father was the youngest son of the 5th Earl De La Warr and a younger brother of the 1st and 2nd Barons Sackville.

On 18 December 1889, Sackville-West was commissioned into the King's Royal Rifle Corps as a second lieutenant and participated in the 1891 Anglo-Manipur War (for which he was mentioned in dispatches)[2] and the 1892 expedition to Burma.

[8] Following his return from the war, Sackville-West was in October 1901 appointed Aide-de-camp to Lieutenant-General Sir Henry Hildyard, Commanding the 1st Army Corps.

[14] In 1906, Sackville-West was attached to the Staff College, Camberley, receiving an appointment as a deputy assistant adjutant-general with the temporary rank of lieutenant-colonel on 21 December.

[20] However, the need for experienced officers in the field prompted his movement to the Indian Army Corps on the Western Front until 3 December, when he was given command of the 21st Infantry Brigade, then part of the 7th Division, and promoted to temporary brigadier general, taking over from Lieutenant Colonel Ralph Berners.

Within days of this posting, however, Sackville-West was wounded in the jaw when a high-explosive shell detonated in the midst of his command group as he inspected the trenches in front of Hamel.

Although wounded, dazed and partially buried, Sackville-West was able to reach the casualty clearing station unaided, and was again evacuated to Britain to recover.

On 19 February 1918 Wilson became chief of the imperial general staff (CIGS) at the War Office, and Sackville-West succeeded him as British military representative at Versailles.